The present invention relates to a ferroelectric film formation solution for forming a ferroelectric film on a substrate and a method for forming a ferroelectric film using the solution. Particularly, the present invention relates to a ferroelectric film formation solution which has neither crystallization nor gelation when forming a ferroelectric film containing a II group element in the periodic table such as Mg, Ca, Sr, or Ba and which can form a stable ferroelectric film on a substrate such as sapphire, titanium, or platinum. The present invention also relates to a method for forming a ferroelectric film using the ferroelectric forming solution.
Lead zirconate titanate (PZT) which is a complex oxide of Pb, Zr, and Ti and lead lanthanum zirconate titanate (PLZT) which is a complex oxide of Pb, Zr, Ti, and La have high dielectric capability, ferroelectricity, piezoelectric effect, and pyroelectricity. The thin films of these complex oxides are therefore used for semiconductor memories such as a DRAM (memories using paradielectrics with high dielectric constant such as (Ba,Sr)TiO3) and FRAM (memories using ferroelectrics such as PZT), capacitors, sensors, and actuators.
In recent years, ferroelectrics containing a small amount of Nb, Fe, Mn, Al, Sn, Bi, Sb, Ba, Ca, Sr, and the like have been studied to improve the characteristics of PZT or PLZT. These ferroelectrics are reported, for example, in Journal of American Ceramic Society, Vol 77, pages 2620-2624 (1994), Japanese Patent Application Laid-Open (JP-A) No. H1-260870, and the like.
Generally, an organic metal compound such as an alkoxide of a metal which is a component of ferroelectrics is reacted in an organic solvent. Then a chelating agent is added to the mixture to improve the stability of a solution, or the mixture is hydrated. And such a solution is widely used for forming the ferroelectrics.
In such a preparing method, however, a solution for forming a thin film of PZT or PLZT containing a II group element in the periodic table such as Mg, Ca, Sr, or Ba tends to crystallize or gel. Even when these phenomena are not produced, the viscosity of the solution changes with time. If the viscosity of the solution increases, the solution cannot be uniformly applied. Also, in the case where the viscosity of the solution increases or decreases, the film thickness when a film is formed varies, giving rise to the problem that it is very difficult to prepare a solution for forming a stable thin film.
An object of the present invention is to provide a stable ferroelectric film formation solution which prevents crystallization or gelation even when forming a ferroelectric film such as PZT or PLZT containing a II group element in the periodic table and which is reduced in a change in the viscosity of the solution with time.
Another object of the present invention is to provide a method for forming a ferroelectric film using the ferroelectric film forming solution.
The present inventors have conducted earnest studies to obtain a stable raw material solution, when forming a ferroelectric film by applying the raw material solution of a ferroelectric followed by drying and baking, which is used for forming the aforementioned ferroelectric film of PZT or PLZT or the like containing a II group element in the periodic table and which neither crystallizes nor gels in a solution condition and is reduced in a change with time in the viscosity of the solution. As a result, the present inventors have found that a stable solution for forming a ferroelectric film of PZT or PLZT or the like containing a II group element in the periodic table which solution neither crystallizes nor gels and is reduced in a change with time in the viscosity of the solution can be obtained by adding acetylacetone and an aqueous nitric acid solution to a solution prepared from an organic solvent and an organic metal compound such as an alkoxide of a metal which is a component of PZT or PLZT containing a II group element in the periodic table.
Accordingly, a ferroelectric film formation solution according to the present invention comprises acetylacetone and an aqueous nitric acid solution in a solution prepared from an organic solvent and an organic metal compound of a metal constituting a ferroelectric.
In this case, the solution prepared from an organic solvent and an organic metal compound means a solution comprising an organic metal compound and an organic solvent, for example the solution in which an organic metal compound is dissolved in an organic solvent or a reacted product is dissolved in an organic solvent, which reacted product is produced by an alcohol exchange reaction (a reaction between an organic metal compound and an organic solvent), a complex alkoxide formation reaction (a reaction between an alcohol exchange reaction product and the organic metal compound and a reaction between organic metal compounds), or coordination of the organic solvent to a metal atom.
Particularly when the aforementioned organic metal compound contains an organic metal compound of a II group element in the periodic table, crystallization or gelation tends to be caused. In the present invention in which acetylacetone and an aqueous nitric acid solution is added to the ferroelectric film forming solution, however, crystallization or gelation can be prevented, which is particularly preferable.
As the II group element, at least one element selected from the group consisting of Mg, Ca, Sr, and Ba may be used.
It is particularly preferable when the aforementioned organic metal compound comprise organic metal compounds respectively containing titanium, zirconium, lanthanum, and lead constituting lead zirconate titanate (PZT) or lead lanthanum zirconate titanate (PLZT), and further an organic metal compound of a II group element in the periodic table to prevent crystallization or gelation which is easily caused in these organic metal compound.
A method for forming the ferroelectric film of the present invention comprises (a) a step of preparing a solution containing an organic solvent and an organic metal compound of a metal constituting a ferroelectric and mixing acetylacetone and an aqueous nitric acid solution with the above solution to prepare a ferroelectric film formation solution, (b) a step of applying the ferroelectric film formation solution to the surface of a substrate, and (c) a step of baking the applied solution after it is dried.
It is particularly preferable when an organic metal compound of a II group element in the periodic table is contained in the above solution to form a ferroelectric film containing the II group element in the periodic table, because crystallization or gelation is not caused and a stable ferroelectric film containing the II group element in the periodic table can be formed.
When organic metal compounds respectively containing titanium, zirconium, and lead which constitute lead zirconate titanate (PZT) are used and further an organic metal compound of a II group element in the periodic table is added, there is obtained a PZT thin film to which the II group element is added.
When organic metal compounds respectively containing titanium, zirconium, lanthanum, and lead which constitute lead lanthanum zirconate titanate (PLZT) are used and further an organic metal compound of a II group element in the periodic table is added, there is obtained a PLZT thin film to which the II group element is added.